This invention relates to a high-definition televison system in which the scanning rates and directions are selected for ease of transcodeability with low distortion to the most common scanning standards for standard-definition television.
The principal world standard scanning rates are 525 lines-per-frame, 60 frames-per-second as used in the Americas and in Japan and 625 lines-per-frame, 50 frames-per-second as used for PAL and SECAM systems in Europe.
U.S. Pat. application Ser. No. 230,384 filed Feb. 2, 1981 in the name of K. Powers describes a proposed component standard for digital television systems which is readily transcodeable to the two major composite systems (NTSC and PAL) by virtue of selecting the common number of pixels in the active portion of each line to be equal to 704.+-.16N, which makes it possible to perform pixel sample-rate interpolation by the use of multipliers consisting exclusively of shifts-and-adds. The digital standard thus proposed provides ease of transcoding between pixel sampling rates of the digital component standard and the preferred sampling rate of the major television systems.
In the production of motion pictures for both theatre and television use, substantial flexibility and cost savings in the editing (post-production) process can result from the use of television-like cameras for generating video signals representative of the image to be viewed, and by use of video tape rather than photographic film for interim storage of each day's production. This process, known as electronic cinematography, is a fast-growing art and can be expected to largely supplant the use of photographic film, at least in the editing phase of production in which different portions of the production are combined and deleted to form the ultimate work of art. Film-to-tape conversions are already widely used in post-production work because of the ease of editing the work in the form of video tape rather than as photographic film.
Present standard-definition television (SDTV) equipments which are based upon the aformentioned 525- and 625-line scanning standards are inadequate to reproduce the full vertical and horizontal resolution limits of 35-and 70-millimeter film. Consequently, a high-definition television (HDTV) system is desirable which has a resolution commensurate with the resolution of the photographic film onto which the images may be impressed.
The aspect ratio of cinematographic (movie) film is greater than the aspect ratio of the present-day standard television systems. Present television systems have an aspect ratio of 1.33 (3 units high, 4 units wide), while aspect ratios of 1.67 and 1.85 are common in cinema use, and ratios larger than 2.0:1 exist. Thus, a high-definition television system to be used for electronic cinematography must have an aspect ratio equal to that of the film to which it is to be transcoded.
In order to be compatible with photographic film, the HDTV should have a resolution in excess of one thousand lines. The number of scanned elements, or to be more exact, the number of scanning lines within the active portion of the picture should be selected to provide lowest-cost transcoding between the HDTV standard and the nominal 485 and 575 active lines of the 525- and 625-line SDTV standards.
When transcoding from one set of lines-per-frame to a second set, aliasing can occur which results from sampling vertically at a rate lower than one-half the resolution bandwidth. This may be avoided by the use of an anti-alias optical prefilter, or by an electronic vertical aperture filter applied to the signal after scanning at a sufficiently high rate. In the latter case, prefiltering by an amount suitable for the lowest expected number of lines per frame, will degrade those transcodings which are to a system having a number of lines greater than the lowest. It is not a well-publicized fact but the vertical resolutions actually achieved in 525- and 625-line systems are approximately equal. Television cameras developed for both markets use the same optics, have the same size and lag characteristic of the image pickup tubes, which are the resolution determining factors. The advantage in subjective quality achieved in 625-line systems over 525-lines is not in increased vertical resolution but rather lies in the lower visibility of the scanning lines and interlace artifacts and in greater headroom with respect to the vertical Nyquist rate and its concomitant lower vertical aliasing in regions of high vertical detail. This subjective quality advantage is counterbalanced by a higher susceptibility to flicker perception from high brightness displays of the 625-line 50 field/second television pictures.
Beats can occur in the event of line-rate transcodings. Conceptually, these result from a slight physical skew of the incoming horizontal lines to be transcoded relative to the outgoing new interpolated lines. This may result for example, from improper synchronization of line counters which allows a line-to-line slippage of the interpolation. In the case of conversion from the HDTV to cinematographic film by laser writing, beats can occur when reading the film with a telecine apparatus. It is desirable to perform such conversions in a manner which avoids such beats.
In a high definition television (HDTV) system, the data rate is much higher than the data rate of the standard-rate-signals to which transcoding is done. In order to conserve memory, it is advantageous to perform the high-definition-to-standard definition transcoding in such a manner that such memory as may be required to perform the transcoding is loaded at the standard-definition rate and not at the high-definition rate.